Low-emission combustion chamber for gas turbine engines

ABSTRACT

A low-emission combustion chamber for gas turbine engines comprises an outer casing with an upstream end wall with a pilot fuel injector, a first flow swirler, an igniting members for initiating a stable diffusion frame in a pilot zone, at least one second coaxial swirler, main fuel injectors, secondary air inlets, and a main combustion zone. For obtaining a still further reduced emissions of primarily nitrogen oxides, the pilot zone is confined radially outwardly by a surrounding wall which constitutes the radially inner confinement of an axial outlet portion of a radial vaporization channel within the second swirler and a third radial flow swirler is adapted to supply the secondary air in a rotary motion opposite to that of the main flow of fuel and air.

FIELD OF THE INVENTION

The present invention relates to a low-emission combustion chamber forgas turbine engines comprising an outer casing with a closing upstreamend wall in which is mounted a pilot fuel injector. Spaced coaxiallyaround the mouth of the injector is mounted a first radial flow swirleradopted to bring air radially entering therethrough to rotate around thelongitudinal axis of the combustion chamber and to be mixed withinjected pilot fuel and the mixture to be ignited by an igniting meansto initiate a stable diffusion flame in a pilot zone. At least onesecond coaxial swirler is being arranged radially outwardly of the zonefor bringing primary air radially entering through the second swirlerand intended for the main combustion, to rotate around the longitudinalaxis and to be mixed with fuel from main fuel injectorscircumferentially spaced around the second swirler. To thisfuel-air-mixture second air is then added for finishing the combustionin a subsequent main combustion zone.

BACKGROUND OF THE INVENTION

Gas turbine engine combustion chambers are previously known from e.g. WO92/07221 and U.S. Pat. No. 4,069,029. Recently it has become still moreimportant not only to reduce the emissions of carbon monoxide andunburnt hydrocarbon from combustion engines but also the emissions ofnitrogen oxide. Particularly for reducing the last-mentioned a veryexact and sensitive control of the entire combustion process in thecombustion chamber is required. A large amount of various measures anddesign improvements have been suggested which imply considerablereductions of the harmful emissions of the engines but in the nearfuture the limit values for the emissions will be further loweredstepwise and therefore still more refined control measures for thecombustion process are now required. The techniques known up to now donot provide for this and therefore further improvements are necessary.

SUMMARY OF THE INVENTION

The object of the present invention therefore is to suggest alow-emission gas turbine combustion chamber of the kind referred to, inwhich a still further improved combustion process can be obtainedthereby provide for still more reduced emissions, particularly ofundesirable nitrogen oxides. According to the invention this is now madepossible by the fact that the pilot zone is confined radially outwardlyby a surrounding wall which at the same time constitutes the radiallyinner confinement of an axial outlet portion of a radial vaporizationchannel located inwardly of the second swirler and adapted to providethe vaporization of the injected main fuel, and because a third radialflow swirler is located axially approximately at the level of thedownstream edge of the pilot zone wall and adapted to supply in a mixingzone the secondary air in a rotary motion opposite to that of the mainflow of the fuel and air around the longitudinal axis.

In the two above-mentioned patent specifications, as a basic measure inorder to reduce particularly the emissions of NO_(X), the step has beentaken to divide the combustion process into several stages axiallyfollowing after each other. By a detailed control of each single step ithas been considered that the combustion could be better controlled andas the result the emission of harmful components reduced. By supplyingthe air required for the combustion in several steps the combustiontemperature can be kept relatively low which is a basic prerequisite forlow emissions of nitrogen oxide.

The present invention, however, is based on the concept that as farupstream as possible in the combustion chamber there is to provide sucha complete and homogenous mixture of fuel and air ignited by an exactlycontrolled combustion process in a pilot zone, that the combustion iscompiled at still at a relatively low combustion temperature within themain combustion zone without division into several axially separatedstages.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, the invention will be further described below withreference to the accompanying drawing in which FIG. 1 is a longitudinalsection through an inventive combustion chamber and FIG. 2 is across-sectional view through the combustion chamber taken along the lineA--A in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings, the low-emission combustion chamber accordingto the invention comprises a pilot fuel injector 4 centrally mounted ina wall 22 which closes the upstream end of a surrounding outer casing21. The casing 21 can be of cylindrical shape or have a can-annularshape in which a plurality of combustion chambers are arrangedcircumferentially spaced around a central axis. Spaced around the mouthof the pilot fuel injector 4 a first swirler 1 is coaxailly mounted. Thefirst swirler is adapted to bring air flowing inwardly radiallytherethrough from the surrounding area closest inside the casing 21 andthe end wall 22 to rotate around a combustion chamber longitudinal axisX. Pilot fuel injected in a known manner through the injector 4 is mixedwith the rotary air and ignited by means of an igniting means 7 forinitiation of a stable diffusion flame in a pilot zone 5.

Radially outwardly of the pilot zone 5 is located at least one secondcoaxial radial flow swirler 2 through which the primary air isintroduced for the main combustion, and which then also is brought torotate around the longitudinal axis X of the combustion chamber. At theswirler 2 are mounted main fuel injectors 13 and to the fuel-air-mixturethus obtained is then added secondary air and the combustion is finishedin a subsequent main combustion zone 6.

According to the invention, the pilot zone 5 now is radially outwardlyconfined by a surrounding wall 23 which at the same time constitutes aradial inner confinement of an axial outlet portion 11 of a radialvaporizing channel 9. The channel is located internally of the secondswirler 2 and adapted to provide a vaporization of the main fuel fromthe injectors 13. According to the invention a third swirler 3 isfurthermore adapted to supply secondary air from the surrounding areaclosest inside the outer cases 21 and end wall 22. The swirler 3 islocated axially approximately at the level of the downstream edge of thepilot zone wall 23 and the vanes are arranged such that the flow ofsecondary air is given a rotary motion opposite that of the main flow offuel and air around the longitudinal axis X in a mixing zone 12.Suitably, the third swirler 3 is mounted on an annular end wall 25 of aflame tube 24 which surrounds the main combustion zone 6. As is evidentfrom FIG. 2 each of the vanes of the second swirler 2 has a crosssectional shape like a wedge or a triangle with one side located on theouter peripheral contour of the swirler with and the other two sidesrunning out into an internal sharp edge.

For introduction of air into the boundary layer at one of or both theradially directed walls 26 carrying the vanes of the second swirler 2and therefore a reduction of the flow friction thereagainst smallapertures 15 might be made in the walls for the introduction of air.

After finished combustion in the main combustion zone 6 the exhaustgases continue their motion outwardly of the Figure and into theturbine.

The advantages of the combustion chamber and the operational mannerthereof are the following. The pilot zone 5 allows that in operation thecombustion in the main combustion zone 6 can be initiated andstabilized. Although the pilot flame is not required as such in order tostabilize the combustion in the main combustion zone the combustion canbe made under leaner conditions and this is of course advantageous inmany cases from the emissional point of view. Another advantage of thepilot zone 5 is that a reliable ignition can be obtained even in lowfuel-and-air proportions in total, which is extremely important incertain engine applications. The location of the pilot zone 5 within thecombustion chamber further implies that the igniting means or spark plug7 can be mounted from the end wall which also is the case with the fuelinjectors and this provides for good accessibility and thereforesimplified maintainance. If required the wall 23 which confines thepilot zone 5 can be provided with film cooling by introduction of airthrough a cooling gap 30.

The vaporization channel 9 consists of three portions, namely a firstradial portion 10, an axial portion 11 connected therewith and a thirdportion 12 for introduction of air from the third swirler 3. Into theradial portion 10 liquid fuel is injected fuel from the main fuelinjectors 13. In the radial portion 10 the air is heavily rotated by thepower impulse from the vanes of the swirler 3 and carry the fueldroplets along, the heavy rotation is a known manner subjecting thedroplets to a continuous acceleration outwardly from the center, whichis counter-balanced by an aerodynamic force directed towards the center.At a selected critical droplet diameter a perfect balance is obtained.Should the droplets be smaller than the critical diameter, they will betransported radially inwardly and out into the axial portion 11 of thevaporization channel. Should the droplets be greater, the inertia forceswill be predominant and the droplets then will be transported radiallyoutwardly and finally hit the edges 14 of the vanes of the swirler 2.There the liquid fuel will be retarded and form a film of liquid whichsuccessively is transported outwardly to the edges of the vanes. Whenthe fuel film reaches the edges, it will be disintegrated again intosmall droplets by heavy shear against the rapid flow of air between thevanes. As a result the fuel droplets will be brought to stay within theradial portion 10 of the vaporization channel till they have beenvaporized or disintegrated into a diameter which is smaller than thecritical. The result thereof is that the fuel can be vaporized duringshort residence times for the gaseous part of the fuel-air mixture atlow and high air temperatures, respectively, which is advantageous sinceit is important to avoid spontaneous ignition of the mixture at the sametime as the fuel still must manage to be vaporized. This pre-mixture canthus be made lean.

In the subsequent axial portion 11 of the vaporization channel then thevaporization is then completed of such droplets which are smaller thanthe critical droplet diameter. The gas flow in the portion 11 alsoassists in cooling the partition wall 23 from the pilot zone 5.

Finally, the fuel-air mixture is mixed into correct stoichiometric valueby supply of air from the swirler 3, this air not only diluting themixture but also giving the same such a turbulent motion that possibleinhomogenities in the fuel-air distribution from the exit of the axialchannel portion 11 will be equalized.

In the above, the combustion chamber has been described in connectionwith the use of liquid fuels. However, it is also possible to useinjectors or spreaders for gaseous fuels such as natural gas whichprovides for the use of the low-emission combustion chamber both forgaseous and diesel fuels with continuous interchanges therebetweenduring operation. Gaseous main fuel then is injected at about the sameposition at the swirler 2 as for liquid fuel but by a larger number ofspreaders since no equalizing effect can be obtained by two-phase flow.

We claim:
 1. A low-emission combustion chamber for gas turbine enginescomprising an outer casing with a closing upstream end walls, a pilotfuel injector mounted therein a first radial flow swirler mounted spacedcoaxially around a mouth of the injector and adopted to bring airradially entering therethrough to rotate around a longitudinal axis ofthe combustion chamber and to be mixed with injected pilot fuel, anigniting means for igniting the mixture to initiate a stable diffusionflame in a pilot zone, at least one second coaxial swirler arrangedradially outwardly of said zone for bringing primary air radiallyentering through said second swirler and intended for the maincombustion, to rotate around said longitudinal axis and to be mixed withfuel from main fuel injectors circumferentially spaced around saidsecond swirler, to which fuel-air-mixture is then added secondary airfor completing the combustion in a subsequent main combustion zonewherein the pilot zone is confined radially outwardly by a surroundingwall which also constitutes a radially inner confinement of an axialoutlet portion of a radial vaporization channel located inwardly of saidsecond swirler and adapted to provide the vaporization of the injectedmain fuel, and wherein a third radial flow siwrler is located axiallyapproximately at a level of the downstream edge of said pilot zone walland is adapted to supply in a mixing zone said secondary air in a rotarymotion opposite to that of the main flow of fuel and air around thelongitudinal axis.
 2. A combustion chamber according to claim 1 whereineach of the vanes of the second swirler has a wedge-like or triangularshape in cross section with one side at an outer peripheral contour andthe other two sides running out into a sharp edge.
 3. A combustionchamber according to claim 2 wherein the third swirler is located at theupstream side of an annular end wall of a flame tube surrounding themain combustion zone.
 4. A combustion chamber according to claim 1wherein in at least one of the two radially directed walls which supportthe vanes of the second swirler are arranged small apertures for theintroduction of air into a boundary layer of the wall and thus areduction of the friction thereagainst.
 5. Combustion chamber accordingto claim 2, wherein in at least one of the two radially directed wallswhich support the vanes of the second swirler are arranged smallapertures for the introduction of air into the boundary layer of thewall and hence a reduction of the friction thereagainst.
 6. Combustionchamber according to claim 3, wherein in at least one of the tworadially directed walls which support the vanes of the second swirlerare arranged small apertures for the introduction of air into theboundary layer of the wall and hence a reduction of the frictionthereagainst.